Not as much as you would think, megapixels or native resolution plays a small role in the value of an image as would any other feature. What we need to do is differentiate Resolution from Quality, or in a sense Actual Resolution, from Spectral resolution.

Why is resolution important?

Most buyers will either be using a stock image for print, advertisement or some other form of commercial publication. The higher the resolution generally the better (in a perfect world). Let's take my camera for example, a Canon Digital Rebel (the old one); the camera features a 6.3Megapixel sensor, giving you roughly an image with 3,072 pixels across, and 2,048 pixels vertically. Most stock agencies and advertising firms request their images to be printable at 300 DPI (dots per inch); this is considered by most to be the professional standard. So let us figure out how large of a print can be achieved from this camera. It's really quite simple, to divide the number of pixels by the number of dots desired per inch.

Now we know that a 6.3Megapixel image will give you roughly a 10.24 x 6.8 image output, barely enough to fill the whole page of a magazine at letter size, most of the time when an images will be used for ads the print will be far smaller than the full page of a magazine. 300DPI is a standard guideline for most professional uses, you will fine that some clients and just every day art admirers might have no problem with an image printed at 150DPI (which would give you roughly 20.48" x 13.65" print resolution).

One important thing to keep in mind that the actual resolution of an image is the resolution actually captured by the camera, do not think that by resizing an image in Photoshop to come out with a larger image, that you will in effect increase the image's printable resolution. Scaling up an image to print at 20.48" x 13.65" @ 300 DPI will yield the same quality if not worse than keeping the image at its native resolution and printing at 150 DPI, since the information to fill in for those other 150 dots per inch has to be guessed by the computer when you are resizing.

When is a 6Megapixel camera better than a 12Megapixel camera?

Despite that a 6Megapixel camera can produce images capable of roughly 10x6 inches of printable resolution; this does not necessarily mean that a camera yielding 8 or even 12 megapixels will produce a better image at the same dots per inch. This concept also applies the same as scaling up that I had described in the last paragraph.

To understand why a higher megapixel camera can often times not produce any better of a result of a lower megapixel camera we need to understand brief amount information regarding sensor technology, or even about film for that matter.

Lets go back to basics for a moment, a standard 35mm film negative consist of an area 36 x 24 mm. That is roughly the dimension that light covers when used in most 35mm film cameras (or of some full frame digital SLR cameras). Film is a little funny when compared to digital sensors, since film for the most part relies on chemical reaction to light. With the size of a 35mm negative something like the more expensive Velvia 50 film theoretically has enough detail per area, that if scanned in with the proper equipment can produce a 25Megapixel image. Most off the shelf film will produce around 3 to 6 depending on the brand, speed and type, but also the type of scanner used, and more or less depending on how correctly the image was captured. Most labs will autocorrect problems such as underexposure, white balance, and red eyes to the point that most day shooters never really notice their own flaws.

Now let us get back to the digital portion of this answer. Now we already understand that film negative is roughly 36x24 in size, and normally called 'Full Frame'. Then there are standard consumer digital cameras, which are usually called Point and Shoot. You can normally identify a consumer camera by the fact that it has a fixed lens system, and that the sensor is always exposed. A way to know is if you can preview an image on the LCD prior to actually shooting the subject. The size of a consumer digital's sensor is roughly 2/3" which is approximately 8.8 x 6.6 mm; that is nearly four times smaller the size of a standard 35mm negative.

Digital SLRs for the most part usually have a larger sensor; the Nikon D70 that you have for example has a sensor size of 23.7 x 15.6 mm. Because most Digital SLR cameras uses the same 35mm lens that their film counterpart use, as opposed to having a fixed lens specifically for that sensor size, digital SLR have crop factor attached to them. Crop factor is a ratio to give consumers a better idea of how the focal length or results will be affected by the camera. In this case the Nikon D70 has a 1.5x crop factor. You can use crop factor as a quick way to judge how close a sensor is to full frame size. Most Canon DSLRs are 1.6x until you get into the professional series (1.3x and full frame). There are even some that are closer to your standard consumer digitals such as the Olympus E-300 that features a 2x crop factor.

Now why is understanding about the crop factor and sensor size useful? Well let us use an analogy to demonstrate. Let us say that we want to use buckets to capture rain, assuming now that the buckets represent the individual photosites or pixels (with color and other technical details excused for the time being), and rain representing passes of light. Let us say that we have an area of land roughly 4 foot across on both sides. I want to setup 16 buckets in this area, then right next to it in the same area of land I want to setup 64 buckets. In order to fit 64 buckets into the same area I must shrink the size of each bucket, this is preciously what happens when you have two cameras of the same sensor size, but with one increased in the amount of megapixels it can capture. Also keep in mind that a consumer 8Megapixel camera has a smaller sensor size than a Digital SLR with 6Megapixel. Continuing forward, when the rain starts coming down, the smaller buckets will start to overflow sooner. The overflow can be represented by noise &grain, as well as hot pixels commonly seen on digital cameras. To eliminate the problem you either have to increase the area for the bucket to allow for more water to be contained before overflow, or to decrease the amount of buckets used.

This is why most consumer cameras generate much more noise and hot pixels in their images as compared to cameras with larger sensors, it could also be true to say that a consumer digital camera with lower resolution in some cases may be a better purchase than someone going out and buying the 12 megapixel model.

Why is this important to know

Back to the original question, about salability. If you took a picture from a digital camera at 6 megapixels and then uploaded it, hoping it's size would sell, then you got an another individual using a camera with a similar or smaller sensor then yours that can do 12 megapixels. It is true that the 12 megapixel image will print at a higher resolution, but the quality at full detail will be inferior to that of the 6 megapixels image.

A Disclaimer

In general practice most of the information here would hold true. One thing to keep in mind is that as time goes by, the technology used in digital cameras become increasingly complex, correcting issues such as noise in certain areas and quality in others. The answers here are to help you get a general understanding that while resolution does play some role in an image's appearance, it is better to have overall quality than resolution.